[Change of offset after implantation of hip alloarthroplasties].

The purpose of the present study was to evaluate the change of the offset after implantation of hip alloarthroplasties. In an experimental study the X-ray templates of 90 different implants (594 different sizes) were digitized (cup and stem) and virtually implanted in the hip joint of 50 consecutive patients who were on the waiting list for hip replacement. Implantation was performed on AP X-rays paying heed to the fit of the shaft and adequate leg length. In total 4500 implantations were performed. After virtual implantation the change of the offset was calculated. There was a wide variance in the ability to fit the implants to the individual patient. For some patients it seems to be extremely difficult to adapt the implant without changing the offset. Some implant designs were only suitable for 2 patients while other designs were suitable for 40 patients (mean: 17). The average change of the offset was 0.27 cm. Considering only the cases in which a change occurs this value increases up to 0.56 cm.For the individual patient the average number of implants that were suitable was 30 (range: 1-67) of a total of 90 designs. Here the average change of the offset was 0.4 cm (-1.04 -- +1.54 cm). Considering only the cases in which a change occurs this value increases up to 0.64 cm. In the majority of the cases the presently available implants do not allow anatomic reconstruction of the individual offset.